The present invention generally relates to an optical repeater provided in an optical communication system, such as an optical submarine (undersea) repeater. More particularly, the present invention relates to an optical repeater having a phase inversion circuit designed to invert the phase of an optical input signal in digital form and output an optical signal having the inverted phase.
The reduction of a transmission loss of an optical fiber makes it possible to increase an interval at which adjacent repeaters are arranged, so that a long-distance transmission of optical signals can be easily realized. In such a long-distance transmission of optical signals, there is a problem of systematic jitter accumulation arising from an arrangement in which an optical signal passes through a plurality of repeaters. Thus, it is necessary to reduce systematic jitter accumulation, particularly in the long-distance transmission.
Referring to FIG. 1, there is illustrated a conventional optical repeater. The optical repeater shown in FIG. 1 is composed of an opto-electric converter 2 connected to an optical transmission line 1, an equalizing amplifier 3, a timing extraction circuit 4, a D-type flip-flop 5, a driving circuit 6, an electro-optical converter 7 coupled to an optical transmission line 8, a frequency divider 9 and a bandpass filter 10.
An optical digital signal via the optical transmission line 1 is converted into an electrical signal by the opto-electric converter 2. The equalizing amplifier 3 amplifies and equalizes the electrical signal from the opto-electric converter 2. An output signal of the equalizing amplifier 3 is input to a terminal D of the flip-flop 5 and the timing extraction circuit 4. The timing extraction circuit 4 generates a timing signal from the output signal from the equalizing amplifier 3. The timing signal generated by the timing extraction circuit 4, which is synchronized with the optical digital signal, is applied to a clock terminal C of the flip-flop 5.
The output signal of the equalizing amplifier 3 is latched in synchronism with the timing signal, and a regenerated digital signal is output via a terminal Q of the flip-flop 5. The reproduced digital signal is input to the driving circuit 6 and the frequency divider 9. The driving circuit 6 drives a laser diode provided in the electro-optical converter 7, which outputs a regenerated and amplified optical digital signal to the transmission line 8. On the other hand, the frequency divider 9 has a flip-flop, which generates a signal having a frequency half the frequency of the signal output from the flip-flop 5. The signal generated and output by the frequency divider 9 passes through the bandpass filter 10, which generates a supervisory signal.
Normally, the optical digital signal is transmitted via the optical transmission lines 1 and 8 in the form of a 24B1P code. The 24B1P code is composed of 24 data bits and a one-bit parity P relative to the 24 data bits. One block is formed of these 25 bits. Normally, if the initial state of the flip-flop of the frequency divider 9 is equal to "0" and no bit error occurs, the output signal of the flip-flop is always equal to "0" at the end of one block, because the number of "1" is even. On the other hand, if the initial state of the flip-flop of the frequency divider 9 is equal to "1" and no bit error occurs, the output signal of the flip-flop is always equal to "1".
Referring to FIG. 2-(a), symbol BL denotes one block, and T denotes a predetermined period during which a plurality of blocks, each having the even parity, are transmitted. Odd parities are inserted at intervals corresponding to the predetermined period T. That is, one odd parity is inserted for every period T. The output signal of the frequency divider 9 is inverted each time the odd parity is applied thereto. In other words, the flip-flop of the frequency divider 9 changes its status each time the odd parity is applied thereto (for every period T). The bandpass filter 6 has a bandpass range having a central frequency substantially corresponding to the reciprocal of the period T, and extracts signal components in the bandpass range. Thus, it is possible to superimpose a low-bit rate supervisory signal on a high-speed data signal and extract this supervisory signal by the combination of the frequency divider 9 and the bandpass filter 10.
In conventional optical repeaters as described above, the repeated optical signal output from each repeater has the same phase as the optical signal input to each repeater. That is, logic "1" of the input signal is transmitted as it is (logic "1"). In a case where a single-mode semiconductor laser diode, such as a DFB (distributed feedback laser diode), is used, a wavelength shift (charping) occurs at the rise and fall of a driving current applied to the laser diode. An optical signal having such a wavelength shift is affected by the wavelength distribution characteristics of the optical transmission lines 1 and 8, so that variations in the rise and fall of the optical signal on the time base, that is, systematic jitter is caused.
Systematic jitter is caused at each optical repeater, so that systematic jitter accumulation occurs in a long-distance transmission. The existence of systematic jitter accumulation deteriorates the transmission quality. For this reason, there are limits of the bit rate of data and the number of repeaters provided in the optical communication system.